Hillary Clinton has rebounded in the Democratic race, partly by delivering bruising blows to her opponent, but the imminent release of her White House schedules could give Barack Obama fodder for attacks of his own.

Clinton has struggled with questions about the records from her years as first lady that remain unreleased, despite requests filed under US freedom of information laws. No evidence has surfaced thus far to suggest the records would prove embarrassing to Clinton, but the long delay in their release has stoked speculation and prompted Obama backers to challenge her support for government transparency.

During the most recent Democratic debate, Clinton depicted the current administration as complicit in the delay — prompting George Bush's aides to direct blame back at Clinton.

The final word came this week from the national archives, which is clearing Clinton-era records for release to the conservative interest group Judicial Watch. The group has sued to gain access to Clinton's papers from the 1990s and was promised 10,000 pages of her White House schedules in advance of a court hearing scheduled for March 20.

The schedules are a potential treasure trove for Obama. The Illinois senator has hammered Clinton for refusing to release her tax returns, while her aides have fired back at Obama for releasing only one year of his requests for the controversial spending projects known as "earmarks".

The archives told Judicial Watch that Clinton's phone logs would take one to two years to be cleared for public release, a delay the rightwing group called "ridiculous". Its records request was filed three months after the archives began accepting them.

Still, the 1990s documents have remained sealed for nearly two years while a representative for the Clinton family examines them. The group is seeking records from the former first lady's ill-fated task force on healthcare reform via a separate lawsuit.

While the tense Democratic race threatens to rage at least until the Pennsylvania primary on April 22, Republican nominee John McCain has a head start in raising money and lining up supporters.

McCain's efforts to woo the Republican base remain shaky, however. Ten conservative senators chose the hour of Bush's endorsement of their nominee to release a package of hard-line immigration bills — playing up McCain's more liberal approach to the issue.

One prominent Democratic senator responded by driving a wedge between McCain and the right, challenging the nominee to disown his colleagues' proposals.

"[I]t is important for Senator McCain to define his position on this cynical Republican effort," said Robert Menendez, who has endorsed Clinton for president.

"The mainstream of America knows full well that the immigration system is broken, but they also recognize that an unbalanced, narrow approach won't fix it."

A Democratic-leaning political group called the Campaign to Defend America (CDA), organised under the same financial rules as the 2004 pro-Bush group Swift Boat veterans for truth, also joined the fray.

The CDA began airing $1m worth of television adverts yesterday that lambaste McCain as an extension of Bush, dubbing him "McSame".